Influence of cropping sequences on labile carbon and phosphorus fractions in a wheat-based conservation agriculture system

Abstract

While conservation agriculture (CA) has largely been successful in many areas, some reports suggest that certain farmers have not realised the benefits they had hoped for, especially in Africa. The benefits of CA could depend on the cropping sequences involved. This study determined the short-term effects of wheat (Triticum aestivum L.)/ maize (Zea mays L.) and wheat/ soybean (Glycine max L.) cropping sequences on fractions of soil organic carbon (SOC), inorganic phosphorus and other soil quality parameters in the 0–200 and 200–400 mm depths on CA farms. Water-soluble carbon (C) and available phosphorus (P) (NaHCO₃ Pi) were significantly higher and NaOH I Pi was lower in maize/wheat than in soybean/wheat sequences. SOC, extractable P, and NaHCO₃ Pi were significantly higher in the 0–200 mm than in the 200–400 mm depth. Extractable P correlated positively with particulate organic carbon (OC) fractions under both sequences. In addition, soil pH, exchangeable potassium (K), calcium (Ca), magnesium (Mg) and cation exchange capacity (CEC) were significantly higher while acid saturation and Ca:Mg was lower in maize/wheat than in soybean/wheat sequences. The findings imply that short-term cropping sequences do not affect SOC sequestration but a wheat crop preceded by maize could benefit from higher soil pH and labile C fractions making P more available, with the additional benefit of available K in these CA systems.

Keywords:

• carbon fractions
• maize
• nutrient availability
• phosphorus fractions
• soybean

Acknowledgements

The authors are thankful for the contributions of the Production Systems Division technical team for liaising with wheat farmers, and assistance with soil sampling and analysis. We acknowledge the financial support of the Agricultural Research Council, National Research Foundation and the Winter Cereal Trust.